Measuring apparatus



June l0, 1941.-. N, E BONN 2,245,124

'MEASURING APPARATUS Filed April 5, 1939 3 Sheets-Sheet 1 June l0, 1941.

Filed April 5, 1.939 5 Sheets-Sheet 2 f l. /m ld L. m

d i j -9' www# June 1o, 1941. M E BONN 2,245,124

MEASURING APPARATUS Filed April 5, 1939 3 Sheets-Sheet 5 C'aeff/c/ enf y @ya Zine/527%@ Patented June 10, 1941 UNITED STATES PATENT'OFFICE 23 Claims.

This invention relates t a measuring apparatus, usually of the potentiometer tyipe, wherein al resistance carrying an electric current is' calibrated in terms of a potential difference or in terms of any other physical quantity which may be translated into a Ipotential difference. I

One objectof my invention is to render the measurements performed with the aid of such apparatus accurate even when the voltage of the source which supplies current to the potentiometer resistance is subject to wide fluctuation. Other objects of my invention will become apparent from' the description which follows.

My invention relates more particularly to apparatus of the type in which light from an incandescent electric lamp is allowed to impinge upon a photoelectric cell after having been partially absorbed or reflected by a medium the characteristics of which it is desired to measure and it provides for measurements which are independent of the voltage'of the source of current supplying said electric lamp. The invention can be used, for example, in photoelectric colorimeters, turbidi-meters, rellectometers, densitometers, smoke detectors Iand the like.

The development of photoelectric tubes and electric method to replace the more dimcult and slower gravimetric and volumetric methods of chemical analysis.

In case of colorimetry which may be taken as an example, typical lapparatus for measuring the absorption characteristics of a solution consists of an incandescent electric lamp, a transparent container for the solution, a photoelectric cell and a g-alvanometer. Often a filter is added to limit the light to a certain narrow ban-d of the spectrum. The light from the lamp, after passing through the filter and the solution, falls upon the sensitive surface of the photocell and produces an electric current which is a measure of the amount of light which is transmitted Iby the solution. Under conditions of constant intensity of light source, it is possible to -correlate in a quantitative way the magnitude of this current with the concentration of a given substance in the solution. In the case of turbidimeters, the

lquantity measured is the amount of light scattered by the particles in suspension which bears a definite relation to the turbidity of the solution which in turn may be quantitatively related to some important property.

Various makes of apparatus of this general class differ from each other in the method of obtaining scale readings. In some cases the scale is obtained directly from galvanometer deflections. Others use the potentiometer method. Still others obtain the scale mechanically by vmoving a neutral wedge, by varying the size of an aperture or -by adjusting the layer-thickns of the solution. Regardless of the method used, the most important factor which limits the accuracy (and the usefulness) of apparatus of this class is the requirement for constancy of light source. To realize how difficult it is to meet this condition, it is sufficient to call attention to the fact that in a tungsten filament lamp a change of 1 per cent in applied voltage produces a change of about 4 per cent in the light output. This means that in order to obtain an accuracy of 1/2 per cent in photoelectric measurements the supply voltage must be constant within 1A; per cent. For an accuracy of .1 Iper cent the voltage must not vary by more than .025 per cent or by one part in 4000. Constancy of such an order is impossible to obtain over any length of time even with the most elaborate control equipment, particularly in industrial applications.

In order to overcome the difficulties which arise from changes in voltage some workers in this field have resorted to the use of differential circuits in which two photocells are connected in electril opposition to each other. Light from a single lamp is divided into two beams. One beam passes through a standard solution or some other .fixed absorbing medium and falls upon one photocell, while the other beam, after passing through the solution under test, falls on the other photocell. The diiferencein output of. the two cells is taken as a measure of the light absorbed by the test solution. In this arrangement variations in light intensity caused .by voltage fluctuations is supposed to affect the two lphotocells alike, affording more accurate results than can be obtained with a single photocell. This method is based upon the assumption that the two photocells have identical characteristics. Experience, however, indicates that this is never actually realized in practice. Even if two cells are found that are fairly well matched under one set of conditions, they are usually found to differ widely when the intensity of the source is changed or when different filters are used. In addition to being only partially effective in producing the desired results, the two-photocell arrangement is fmore complicated, and apparatus incorporating this feature must have a number of additional adjustments which render such apparatus inconvenient and expensive.

In the arrangement which forms the preferred embodiment of my invention I use but one photocell. By the addition of simple, reliable and inexpensive elements I succeed in rendering photoelectric measuring apparatus substantially independent of voltage fluctuations within Wide limits. My invention thus accomplishes the following objects:

1. It makes possible the attainment of much greater accuracy in photoelectric measurements than has been possible heretofore.

2. It employs simpler, more convenient and less expensive apparatus.

3. It opens the field for wide application of photoelectric measuring methods in industry by point-ing the way to the use of rugged and automatic recording and controlling apparatus based upon the null method.

4. In addition, by the use of circuits devised by me, it renders possible the automatic recording of physical magnitudes where the amount of energy available is too small to operate the actuating mechanism of sturdy recording instruments based upon the null method of measurement.

My invention can be explained in greater detail by reference to the accompanying drawings which show, more or less diagrammatically, several assemblies of apparatus elements with cooperating electrical connections which constitute illustrative embodiments of my invention. vIn this showing,

Fig. 1 is a diagram illustrating schematically the main features of my invention as applied to the measurement of the light absorption of a solution,

Fig. 2 is a schematic illustration of a similar nature as applied to an automatic recording apparatus,

Fig. 3 illustrates schematically the application of my invention to automatic controlling apparatus,

Fig. 4 illustrates an apparatus wherein the scale of transmission values is obtained by varying the thickness of the layer of the solution,

Fig. 5 is a diagrammatic illustration of an arrangement utilizing a light-sensitive element of the photo-emissive type,

Fig. 6 is an illustration of an alternative combination of certain elements comprising the compensating feature of my invention, while Fig. 7 shows an arrangement in which a source of alternating current is used.

In the various figures like elements are designated by like reference numerals. Referring to Fig. 1, electric incandescent lamp 2 has a filament I made of any suitable material such as tungsten. Situated in the path of the light from lamp 2 is absorption-cell 8 made of glass or any other suitable light-transmitting material. Absorption cell 8 contains the solution Whose lightabsorbing properties it is desired to measure. Any light not absorbed by the solution falls upon the face of photo-cell 9 to the terminals of which is connected resistor I 0 preferably of relatively low resistance value and made of a material having a negligible temperature coeicient of resistivity, such as manganin. Photo cell 9 is, in this instance, of the barrier-layer type which has the property of generating an electric current proportional to the amount of light ux incident upon its sensitive surface, provided that the resistance connected across its terminals is not greater than several hundred ohms.

Filament I together with resistors'3, 4, 5 and slidewire 6 form a Wheatstone bridge network. As indicated in Fig. 1 the electrical resistance in ohms of the four bridge arms will be designated by the symbols L, N, A and B, where L is the resistance of lamp filament I, N the resistance oi.' resistor 3, A the resistance of resistor 5 together with that part of slidewire 6 which extends from resistor 5 and slider 1, and B the resistance of resistor 4 together with the remaining part of slidewire 6. The bridge is supplied with current by source I3. The resistances of bridge arms A, B and N are so chosen that for the desired temperature of filament I the bridge is out of balance, so that a potential difference e exists between points a and b. This-potential difference sends a current through rheostat II and potentiometer I2. The drop of potential across the whole of potentiometer I2 can evidently be regulated by adjusting rheostat II.

The positive end of potentiometer I2 is connected by lead I4 to the positive end of resistor I0, While slider I5 is joined by lead I6 to one terminal of galvanometer I1, the other terminal being joined .by lead I8 to the negative end of resistor I0. When the potential difference between slider I5 and the positive end of potentiometer I2 is equal to the IR drop across resistor I0, no current ows through galvanometer I1, and its index indicates zero. Attached to potentiometer I2 is scale I9 which may be marked in even divisions from 0 to 100i to indicate the relative amount of light absorbed by the solution in absorption-cell 8 or it may beV calibrated in terms of concentration of the constituent of the solution which is the cause of its lightabsorbing quali-ty. or in any other suitable manner.

The potential difference e between points a and b may be expressed in terms of the voltage E applied to the bridge by thefollowing equation:

If all the values in the parentheses were constant, then a change in E would result in a proportionate change in e. In this case, however, a change in E effects a change in L, the resistance of lamp 2, and this causes the relative change in e to be greater than the corresponding relative change in E. The percentage change in e for a given percentage change in E depends upon the relative values of the bridge resistances, in particular upon the value of or upon the ratio of resistances A and B. It follows that'by adjusting this ratio, as by moving slider l, the rate of change of the unbalance potential difference e with respect to total voltage applied to the bridge can be easily controlled. Since the current iiowing through potentiometer I2 is substantially proportional to unbalance voltage e, it follows that by moving slider 'I the rate of change of this current and of the potential difference of potentiometer I2 with respect to voltage E of source I3 is easily adjusted to substantially any desired value.

For a numerical example it will be assumed that filament I is at a temperature of A2700 degrees K., that its resistance at that temperature is 10 ohms and that these conditions are realized volts. The potential dierence between pontsa and b is evidently l Now assume that the voltage of source I3 has' dropped to some lower value so that the voltage -across filament I is now only 9 volts. 'I'hls will cause a considerable drop in the temperature of la'ment I. Since the resistance of a tungsten lament decreases by about 1/2 per cent for each 1 per cent drop in the voltage applied to it, the resistance of lament I is now about 9.5 ohms. At the same time the amount of light emitted by filament I will decrease td about 60 per cent of its original value.V This will cause the current generated in photocell 9 and the drop of potential in resistor I to decrease also to 60 per cent of their former values. Since the resistance of bridge arm N has undergone no change, the voltage of source I3 is now evidently The drop in bridge arm A is now and the potential difference between points a an b is now But it is evident that if the resistance values of bridge arms A and Bare so chosen that the new potential difference between points a and b will now equal 60 percent of the potential dierence originally existing between these points, then the potential dropin potentiometer I2 will undergo the same relative change as the potential difierence across resistor III under a change in voltage of source Il with all other conditions remaining fixed. It follows that compensation is complete and that the indications of the instrument, with the bridge in such adjustment, will be fully independent oi voltagefiuctuations of source I3.

In order to obtain the ratio between bridge arms A and B necessary to effect the desired result, all that it is necessary to do is to solve the following equation which is but a statement in mathematical form of the ideas expressed above.

volts volts equal to 11 ohms.

we find that It is convenient to make the resistance of arm A equal to 95 ohms and the resistance of arm B With such values for arm A and B, there will be perfect compensation for voltage iluctuations of source I3 provided that the assumptions relative to change in resistance and brightness of filament I are quantitatively correct.

While these assumptions and these calculations can be used as a guide in the design of apparatus in accordance with this invention, it is not at all necessary that the exact amounts be known by which the resistance of filament I and the light flux emitted by it change for a given change in applied voltage. In practice, there are incorporated within thel apparatus itself means for adjusting the ratio between bridge arms A and B and means for ascertaining the proper adjustment of this ratio to obtain per cent compensation. With such means at his disposal, the user of the apparatus can himself make from time to time the adjustments which may be required due to changes in the characteristics of lamp 2 with age, or when replacing an old lamp with a new one which may have diierent characteristics. To this end, there is provided within the apparatus, in addition to the elements already described, resistor 20 which is normally snorted by switch 2l operated by push-button 22. The operationis as follows:

With absorption-cell 8 lled with distilled water or some other solvent which may be useful in the particular technique, slider I5 of potentiometer I2 is set on the same zero of scale I9 corresponding to zero concentration of the constituent whose light-absorbing properties are under investigation. Rheostat II is then adjusted until galvanorneter I1 indicates zero. This means that the potential drop across resistor II) equals the potential drop across the whole of potentiometer I2.

Push-button 22 is now depressed. This opens switch 2i and introduces in series with source I3 resistance 20 thus lowering the voltage available for lamp 2 by an amount which depends upon the magnitude of resistance 20. The value of this resistance may be conveniently chosen so that its introduction in series with source I3 reduces the voltage across filament I by about 10 per cent. If upon depressing push-button 22, galvanometer I1 remains at zero, it signifies that compensation is complete, because the change in the current of photocell 9 due to the decrease in light flux from lamp 2 must have been of the same relative amount as the decrease in current ilowing in potentiometer I2 due to change in the unbalance potential of the bridge occurring between points a and b. If, however, as is more likely to be the case, galvanometer I1 deilects when push-button 22 is depressed, this shows that compensation is not fully eiective. In this case, push-button 22 is released and slide 1 of slidewire i is changed in position thus altering the ratio of bridge arms A and B. This will cause galvanometer I1 to deflect. Balance is now restored by adjusting rheostat II. These adjustments of slider 1 and rheostat I I are repeated until the depression of push-button 22 produces no movement of the index of galvanometer I1.

.considered adequate for all changes of voltage across lamp 2 which are no greater than the change produced by depressing push button 22.

'no deflection of the galvanometer needle.

Once these` adjustments are made they need not be repeated for more or less extended periods of time, depending upon the temperature of the lament and the rate of its deterioration. However, it is advisable to check this adjustment once a day by merely depressing push-button 22 and observing the action of galvanometer I1.

An alternative method of adjusting the ratio of bridge arms A and B and rheostat II, to produce voltage compensation can be followed. In this method, after preliminary adjustment of rheostat II, to produce zero deflection of the galvanometer, with slider I5 set at the zero of the potentiometer scale and with resistance 20 shorted, the push button 22 is depressed, throwing resistance 28 in series with the current source I3. This will, in general, cause a deflection of the galvanometer needle. The galvanometer can then be brought to zero deflection by adjustment of slider 1 which changes the bridge ratio. If the button 22 is now released, the galvanometer will, in general, be caused to deflect once more, in which case rheostat I I can be adjusted to produce zero deflection, these operations being repeated until the depression of button 22 produces Of course, after voltage compensation is once attained, only minor adjustments of slider 1 and rheostat II are required in the subsequent use of the equipment, which adjustments can be made very quickly.

Having thus rendered the apparatus independent of fluctuations in the voltage of source I3, the user may now proceed to calibrate the potentiometer'` for various concentrations of the light absorbing constituent, by placing within the absorption cell 8 solutions containing this constituent in various known concentrations and in each case adjusting slider I5 of potentiometer I2 to obtain a balance as indicated by the absence of a deflection in the galvanometer I1, the reading of scale I8 for each concentration being noted. The calibration table or curve thus obtained will hold for future determination of this particular constituent in solutions of unknown concentration.

The wiring diagram shown in Fig. 1 can, of course, be employed in connection with one of the conventional automatic recording and/or controlling devices which are now on the market. For example the potentiometer I2 can be automatically balanced and a record can be made of the potentiometer readings while a solution whose light-absorbing characteristics are to be determined is flowing through the absorption cell 8. Or the slider I5 can be set at a point which represents the characteristic desired in a solution owing through the absorption cell 8. Or the slider I5 can be set at a point which represents the characteristic desired in a solution owing through absorption cell 8, the deflections of the galvanometer I1 being used, either directly or indirectly to control automatically the characteristics ofthe solution to the desired point, as will be readily understood by those skilled in this art and as will become evident from the following description.

The equipment shown in Fig.'1 can evidently be described broadly as comprising a Wheatstone bridge having the usual four arms and a source of current I.3 connected to opposite points of said bridge, a compensating circuit including rheostat II and potentiometer I2 connected across said bridge diagonally with respect to said source of current, anv energy emitting element 2 connected 75 in one of thearms of said bridge, the energy emitted from said element being dependent upon the voltage of said source I3, an energy measuring circuit including an energy-receiving element adapted to receive energy from said energyemitting element, said energy-measuring circuit being interconnected with said'bridge and with said compensating circuit and being normally functionally dependent upon the changes in energy received by said energy-receiving element from said energy-emitting element induced by changes in the voltage of said source, and in-g cluding means (slidewire 6 and slider 1) for adjusting the state of unbalance of said bridge to produce a potential difference in said compensating circuit varying with the voltage of said source of current in such a manner that the measurements are rendered independent of the voltage changes of said sourceof current.

In Fig. 2 I have shown, in a diagrammatic manner, an embodiment of my invention in an apparatus for automatically producing a graphic record of any variations in the color of a solution. This equipment includes all of .the elements of Fig. 1 as well as a number of additional elements for automatically maintaining the circuit in balance and for producing the desired record.

In Fig. 2, absorption cell 8 is preferably lof ,the flow type which receives the solution through inlet pipe 25 and discharges it through outlet`t26. Interposed between lamp 2 and absorption ycell 8 there are shown lens 23 for rendering the light rays parallel and light lter 24 which transmits light only in that part of thespectrum which is most suitable to the particular liquid flowing through absorption c ell 8. The use of light filterl 24 not only increases .the sensitivity of the apparatus to a particular color, butalso simplifies the calibration of the apparatus inasmuch as it may render applicable the well known Bourger- Beer'law of transmission of monochromatic radiation.

To render the apparatus self-balancing the slider I5 of potentiometer I2 is fastened to nut 38 driven` by screw 31 whichis rotated in one direction or the reverse by gear 36 and worm 35 attached to shaft 34 of reversible motor 3I which receives electric current through leads 32 and 33 and is controlled by pointer 21 of galvanometer I1 and contacts 4I and 42 -through leads 28, 29 and 30. Attached to nut 38 and moved by it simultaneously with slider I5 is marker39 arranged to draw a line upon chart 40 which is preferably driven at constant speed at right angles to the direction of travel of nut 38 by a suitable clockwork or constant-speed motor not shown. The chart may carry an arbitrary scale marked 0 to 100 or it may be calibrated in' terms of the density or concentration of the coloring constituent in .the vfluid whose characteristics are being determined.

The operation of the recorder is as follows: With lamp 2 lighted' and with pure solvent (zero concentration of the chromogen) in absorption cell 8, suppose that the drop of potential in resistor I0 isgreater than the potential .difference between the right end of Ipotentiometer wire I2 and slider I5. Galvanometer pointer 21 will be deiiected to the right touching contact 42. This completes the circuit between conductors 28 and 30 and causes motor 3| to start in sucha direction that nut 38 is moved to the left. Galvanometer pointer 21 should return to its neutral position and motor 3| should stop as soon as slider I5 has reached .the left end of potentiometer wire I2. If such is not the case, rheostat II is adjusted until the potential drop across the entire potenitiometer wire l2 just equals the potential drop across resistor I8. Push-button 22 should nowV be depressed and slider 1 and rheostat I'I adjusted, if necessary, to render the apparatus independent of iiuctuations of voltage of source I3 in the manner previously described.

Having made these preliminary adjustments, the liquid whose color variations are to be graphically recorded is allowed to ow through cell 8. The presence of the chromogen in the solution will cause less light of the given wave length to reach photocell 9. As a result less currentwill ow in resistor I0, the electrical balance of the circuit will be destroyed and galvanometer pointer 21 will deect to the left touching contact 4I. This completes the circuit between conductors 28 and 29 and causes motor 3| to start in a direction to move nut 38 to the right. When slider` I5 has reached a point where' the potential difference between it and the right end of potentiometer wire I2 is just equal to the voltage drop across resistor I8, galvanometer pointer 21 will return to its neutral position, thus breaking the circuit between conductors 28 and 29v and causing motor 3| to stop. At the same time marker 39 will have occupied a positioii on the chart corresponding to the actual concentration of the chromogen in the solution. Evidently, with this arrangement any change in the concentration of the chromogen will cause nut 38 to move both a slider |5 and marker 39-either .to the right orto the left, as the case may require, thus automatically maintaining the balance of the electrical circuit and at the same time producing a graphic record of the concentration of the chromogen on the chart 40. The mechanism illustrated in Fig. 2 for automatically balancing the circuit is of an elementary nature. It was purposely chosen in order to facilitate explanation. In practice, other conventional mecha-- nisrn for accomplishing the same result, such as that disclosed in Letters Patent of the United The arrangement of Fig. 3 is in many respects similar to that of Fig. 2, but with these important diierences. In place of potentiometer I2 and slider I5 there is substituted iixed resistance |2. The voltage drop in resistor I2' is manually adjusted to equal the voltage drop in resistor I8 when the characteristic of the liquid corresponds to that desired. Thereafter, any departure from the desired condition results in a deflection of galvanometer pointer 21 which causes motor 3| y 34. Stirrer 45, preferably power operated, serves in optimum amount.

the purpose of thoroughly mixing the material with the liquor. After treatment in tank 43 the liquor is discharged through pipe 49, the rate of discharge being controlled by the setting of valve 58. A small fraction of the treated liquor is diverted from pipe 49 through tube 25 into absorption cell 8 whence it is discharged through tube 25. Into absorption-cell 8 also flows from bottle 54 through -tube 52 a suitable indicator solution.A The rates at which the liquor and the indicator solution are supplied to absorption cell 8 are closely controlled by some suitable means, such as valve 5| and stopcock 53. The mode of operation is as follows:

Filter 24 is selected to have maximum transmission in that part of the spectrum which is most suitable to the lindicator in use. With valve l5| and stopcock 53 closed and motor 3| disconnected from the power supply, absorption cell 8 is filled witha sample of the liquor which is known to'possess the desired' characteristic The indicator is'added and rheostat I| adjusted until galvanometer States No. 1,125,699, may be preferred. It is to be understood that my invention comprehendsy the use of any other suitable or equivalent mechanism.

In Fig. 3 there is illustrated, in a, diagrammatic manner an arrangement embodying my invention for automatically maintaining constant a certain light-absorbing characteristic or quality of a fluid when that characteristic can be controlled by the admixture of a. material. The fluid may be such that its own inherent color can serve as an indication of the particular characteristic, or a. color indicative of this characteristic may be developed for test'purposes by the addition of an indicator to test samples. Cases ofthe latter type are particularly numerous. For example, the degree of acidity or alkalinity of liquors used in industrial processes is of great importance.

Indicators are available which when added to the liquor, produce a color indicative oi its pH value. In process control it is customary to obtain at frequent intervals samples of the liquor and, after adding an appropriate indicator, to test for color by ,visual or photoelectric inspection. Because my invention renders photoelectric measurements independent of voltage variations, it now becomes possible to exercise close control over the characteristics of a liquid in an automatic way which is not only more eillcient but less costly.

pointer-21 is in neutral position. Push-button 22 is now depressed to test for the eect of voltage variation of source I3 and, if necessary, slider 1 and rheostat |I are adjusted to render the system indiierent to such variations, as already explained. Valve 5I and stopcock 53 are now opened to the required extent and motor 3| joined to the power supply through leads 32 and ,33. As long as the liquor flowing out of I tank 4a has the desired characteristic in predetermined measure, photocell 9 will receive Just the correct amount of light to cause it to generate the current required to make the potential difference across resistor I8 equal to the vpotential diierence across resistor I2' regardless of possible fluctuations in the voltage of source I3.

- Under these conditions, galvanometer pointer 21 i will remain in a neutral position and motor 3| at a standstill. of the liquor changes, either more or less light of the given wave length will reach photocell 9, causing its current output to increase or decrease respectively, with the result that a change occurs in the voltage drop across resistor |2'. 'Ihis will cause the galvanometer pointer 21 to move from its neutral position in one or the other direction,'as the case may require, completing the energization of motor 3| and causing it to adjust the position of valve 48 to increase or decrease, as the case may require, the'supply of the characteristic controlling material to tank If however, the characteristic 43. When the rate of feed of this material is just right to produce the desired characteristic in the correct amount, the circuit will become balanced, galvanometer pointer 21 .willy return to its-'neutral position and motor-3l will stop, leaving valve 48 in correct adjustment corresponding to the new conditions..

An arrangement for obtaining scale readings by varying the layer thickness of the solution is illustrated schematically in Fig. 4, Here the light from lamp 2 is directed by lens 23 through iilter 24'upon plunger 55 made of optical glass or other suitable material. After passing through plunger I55 andthe solution contained inY absorption cell 8, the unabsorbed light falls uponthe sensitive surface of photocell 9. Plunger 55 is rigidly connected by member 56 to nut 51 t vanometer I1 is placed key 62 which may be of the lock-down type and which serves the purpose of protecting galvanometer llagainst violent -deflections which may take place when the circuit is greatly unbalanced.

To calibrate such apparatus for use in a given technique, the following procedure may be followed. A number of solutions containing the chromogen in various known concentrations, all in the same solvent, are made up. The solution containing the chromogen in lowest concentration is placed within absorption cell 8 first and pointer 60 brought to zero on scale 6| by adjusting knurled head 59, which gives the .solution its greatest layer thickness. While gently tapping key 62, rheostat II` is adjusted until galvanometer I1 shows no deiiection. Key 62 is now locked down and push-button 22 tapped to observe the eiect of a variation in the voltage of source I3. If necessary, slider 1 and rheostat marker producing a record upon an appropriate y chart.

In 'the several arrangements heretofore 'disclosed the light-sensitive device was assumed to be a photocell of the so called barrier-layer type which generates its own current under theiniiuence of light. My invention, however, is not limited to use with devices of this type'only,y

but can be used with equal advantage in conjunction with photosensitive devices of other I types. An arrangement showing the application of my invention -to a tube operating on the' principle of photo-emission is illustrated in Fig; 5.

Here, phototube 9 contains anode 63 and cathode' 64. Resistance II) and battery 65 are connected in series between these two electrodes. Under the influence of light-from lamp 2 transmitted by the solution in absorption-cell 8 a current will iiow in resistor I6. This current, and consequently the drop of potential across resistor I9 is proportional to the lig-ht flux incident upon cathode 64. The potential diierence across resistor I0 is in opposition to a potential difference derived from potentiometer I'2.

II are adjusted until galvanometer I1 remains v at -zero when push-button 22 is depressed.

Having thus rendered the apparatus insensi- Having once prepared such table or'curve, solutions of unknown concentration may be measured subsequently by placing them in absorptioncell 8 and adjusting plunger 55 to balance the electrical circuit containing resistors I0 and I2. Occasionally it is desir-able to check the adjustment of slider 1 and rheostat I l to make certain that the ageing of lamp 2 has not disturbed the calibration. 1f such effect is noted the corrective adjustments are easily made as previously explained.

While I have shown in Fig, 4 amanually operated apparatus it will be understood by those skilled in this art that this arrangement, like the arrangement oi Fig. 1, also lends itself to automatic recording in a manner similar to that pointer Vthere is provided a suitable pen or Due to the high impedance inherent in phototubes of this type, *the current flowing in resistor Il) is very small.' In some cases it is satisfactory to use a sensitive reecting galvanometer as a balance detector in the connections between resistor I0 and potentiometer I2. In the majority of cases, however, it is preferable to interpose an amplifier between resistor III and potentiometer I2 on the one side, the galvanometer I1 on the other side, as shown in Fig. 5.

Amplier tube 66 contains heater 61, cathode 68. anode or plate 69 and-control element or grid 10. Heater 61 is connected with resistances 1I, 12, 1'3 and slidewire 14 to form a Wheatstone 46 bridge, similar in arrangementto the bridge usedy in conjunction with lamp 2. Cathode 68 is connected to slider 1=5of slidewire 14, while grid 10 is connected through resistor I0, lead I4, poten- ,tiometer I2, slider I5 and lead I6 to the junction point 16 of resistor 1I and heater 61. The bridge, including heater 61, receives current from battery \or othersuitable source 11 through resistors 1-9 and 88. In series with battery 11 there is also connectedresistor 18 which is normally shorted by switch 8l controlled by push- .,button 82.- The purpose of the last three items is to enable the user to test for response of the amplifier to variation in voltage oi' source 11. Connected between plate 69 and junction point 86 vof source 11 and resistor 80 is potentiometer 83 whose sliding contact 84 is connected through galvanometer I1 to junction 81 of resistors 19 and 80. A'convenient operating sequence is as follows:

Wirth source I3 disconnected so that lamp 2 is dark and phototube 9 -reecives nolight, but with all other Aconnections complete as indicated, slider 84 is adjusted until galvanometer I1 indicates zero. This means that the drop of potential between slider 84 and point '86 due to the current iiowing to plate 69 is just equal .'to the drop in resistor which carries the current supplying heater .61 and its associated network. The only voltage now acting in the input circuit of amplifier tube 66, i. e. between grid 10 and cathode 68 -is the potential-diiierence between point 16 and slider 15 which is the unbalance voltage of the bridge associated with heater 61 and which is advantageously negative to aiord a desirable negativebias for grid 10. There is no potential drop e cluding'resistor 80 and galvanometer I1.

in resistor I8 because phototube 9 is dark. Likewise, there is no potential diierence in poten-'- tiometer I2 because source I3 is not supplying manganin. The bridge is excited from the same tial of pl-ate 89 due .the diminished drop in resistors 80, 19`and 13. As a result there will be a tendency for the plate current ilowing through potentiometer 83 to decrease in much greater proportion than the corresponding decrease in the current owing in resistor 80. However, the drop in the temperature of heater 61 will lower its resistance, and will cause the negative unbalance potential difference servingas a negative bias for grid 10 to decrease and thus tend to increase the palte current. By proper adjustment of slider 15, it is quite easy -to c-ause the two eilects to cancel each other and to have a condition where a change in the voltage of source 11 produces no change in the circuit in- Accordingly, the position of slider 15 is adjusted until galvanometer I1 exhibits no movement when push-button 82 is depressed.

It is evident that the compensating effect just described is essentially similar to that previously described in connection with lamp 2.

Having adjusted theamplier so that it is not sensitive to variations in voltage of source 11, source I3 is now connected to the network of lamp 2, absorption-cell 8 is filled with pure solvent, slider I5 is set on the zero of the potentiometer scale I9 and rhecstat II is adjusted until galvanometer I1 again reads zero. This means that the potential drop across the entire-potentiometer I2 is just equal to the potential drop across resistor I due to electron emission from cathode 64 ofl photo-tube 9 caused by radiation from lamp 2. To test the system for freedom from eiiects of volt- -age variations of source I8, push-button 22 should be depressed and, if galvanometer I1 shows a deflection, slider 1 and rhecstat II should be adjusted until the depressing of push-button 22 has no effect upon galvanometer I1. The system is now in complete adjustment to compensate for changes in both source I3 and source 11.

To measure the light absorption of a substance, the substance is dissolved in the same solvent in known concentration, the solution is placed in absorption-cell 8, and slider I is adjusted until galvanometer I1 again reads zero, The absorption or extinction coefficient of the solute may now bel obtained by reference to a previous calibration of scale I8, or, in case of monochromatic light, by the applications of the law of Bourger and Beer previously mentioned.'v

In all the previous illustrations, I have shown lamp 2 as connected in one arm of the compensating bridge. While this arrangement is a convenient one and is preferred'by me, other arrangements embodying my invention maybe desource I3 which supplies lamp 2. 'Ihe electrical resistance of resistor 88 thus becomes a function of the voltage of source I3. By proper choice of resistance values for the various'bridge arms, it is possible to have a condition where a change in the voltage of source I3 produces the same relative change in the potential drop of potentiometer I2 as is produced in the light emitted by lamp 2 by the same cause, thus affording complete compensation.

It will be understood by those skilled in this art that lamp 2 and amplifier 66 of Fig. 5 and their associated networks may be operated from an alternating current source or sources through suitable rectifiers. It is also possible to light lamp 2 with unrectied alternating current, but to rectify the unbalanced voltage of the bridge to furnish direct current to the potentiometer I2. There are several other ways in which alternating current can be employed for lighting the lamp 2 within the scope of the present invention.

In the arrangements shown in Figs. 1 to 6 the Wheatstone bridge network including lamp 2 is shown supplied with current from an individual source, such as a storage battery. Since my invention affords effective automatic compensation for considerable voltage fluctuation, it is evident that the current for the bridge may be derived from any convenient source, such, for example, as the lighting or power mains. In locations where the available commercial supply is tentiometer I2 receives direct current, as required.

, Although any suitable rectier may be used, the one which I have selected as an example in Fig. 7 is of the well known thermocouple type, consisting of heater 89 and thermojunction 90 in good thermal contact therewith. Heater 89 is connected to points a and b of the lamp bridge, while thermocouple 98 is joined to potentiometer slidewire I2 through rhecstat II. An unbalance potential difference existing between points a and b causes a current to iiow through heater 88 which raises the temperature of the junction of thermocouple and generates a direct potential diierence at the terminals of the couple which in turn sends a direct current through rhecstat II and potentiometer I2. By proper selection of bridge resistors and by proper adjustment of the slider 1 it is possible to obtain a condition where a variation in the lvoltage of the source supplying the lamp bridge will cause a variation in the potential diiference across potentiometer I2 which is equal to the change in the current flowing in resistor I0 caused by the resulting change in the light intensity of lamp 2, thus affording complete compensation. 'I'he effectiveness of the compensation may be tested, as before, by introducing a resistance in series with the source, as by depressing button 22.

In the previous disclosure I have confined the description to what I consider to be the most useful embodiment of my invention. However, it is embodiment.` The essential feature of my invention is the combination of an unbalanced Wheatstone bridge containing in one arm a lamp or some other resistor of substantial temperature coecient with a source of voltage to supply current to a calibrated potentiometer resistance, such as is used for purposes of electrical measurement or control, in such'a manner that variations in the voltage of said source do not affect the accuracy of said measurement or control. Justas slider 1 and rheostat I I may be adjusted to cause the curret in potentiometer I2 to vary in accordance with the brightness of lamp 2 under the inuence of variations in the voltage of source I3, so these controls may also be adjusted to maintain the current flowing through potentiometer I2 at a constant predetermined value in spite of variations in the voltage of source I3. In fact, as will easily be perceived by those conversant with this art, the bridge and the controls may be so arranged that an increase in the voltage of source I3 results in Va decrease in the current flowing through potentiometer I2; When the bridge is so adjusted that the potentiometer current remains constant under conditions of varying supply voltage, the combination becomes useful for all kinds of potentiometric measurements, as will be readily understoodby those familiar with the potentiometric art. My invention comprehends al1 such arrangements, applications and modications as come within the letter and the spirit of the following claims.

What I claim' is: ,l I

1. In a system including an energy emitting element and a source of electric current for said element and a receiver for said energy, the combination of a plurality of.electrical resistorsvin a bridge network `to produce a `potential diiference, said bridge being adjusted to render said potential difference functionally dependent upon the voltage of said source, and means for applying said potential difference to another element of said system to counteract an eiect producedv upon said system by changes in emission from said first-named element caused by variations in voltage of said source.

2. In a system including `an energy-emitting element, an energy-receiving element and a source of electric current, lthe combination of a plurality of electrical resistors in a bridge network to produce an unbalance potential difference, said bridge being adjusted to cause variations in the voltage of said source to effect changes in saidpotential difference which are substantially proportional to the corresponding changes in emission from said energy emitting element caused by said variations, and electrical means for applyingsaid potential difference' to a third element ofl said system to render it insensitive to said variations.

3. An electriccircuit comprising an electrically actuated energy .emitting elementwhose resistance and energy emission vary with the current passing therethrough, supplying said element and a receiver for said energy a plurality of electrical resistors connected in a bridge network to produce a potential unbalance, a source of current supplying said bridge and said energy emitting element, said energy emitting element being connected in one of the arms of said bridge in such manner that said unbalance is determined by the current passing therethrough, variable'means for adjusting said unbalance to render said potential difference functionally dependent upon the voltage of said source, and

means for applying said potential difference -to another element of said circuit to counteract an effect produced .upon said receiver by variations in emission from said emitting element caused by changes in the voltage of said source.

4. In photoelectric apparatus including a source of radiant energy, an electric source for supp1ying current thereto .and a photo-responsive vdevice adapted to receive energy from said first named source and to produce an electric current as a result thereof, the combination of a plurality of electrical resistors in a bridge network to produce an unbalance potential difference, a circuit for opposing said potential di`erf ence to a potential difference derived from said photo-responsive device, variable electrical means for adjusting said first-named potential difference to substantial equality with said secondnamed potential difference, and means for adjusting said bridge to cause changes in the voltage of said second-named source to eifect substantially equal variations in said two potential differences.

5. The combination with a potentiometer of a source of electric current, a Wheatstone bridge disposed between said potentiometer and said source and including a resistor subject to variation with changes in thevvoltage of said source, a circuit including a galvanometer forropposing said potentiometer with a potential difference to be measured, regulatory means for adjusting said electric incandescent lamp, an electric source for supplying current thereto and a photo-responsive device adapted to receive radiation from said lamp and to produce an'electric current as a result thereof, the combination vof `a plurality of electrical resistors in a Wheatstone network to produce an unbalance potential dierence, a circuit for opposing said potential difference to aK ,potential difference derived from said photore.

sponsive device and varying with thebtrightness of said lamp, regulatory means for adjusting said network to cause a variation in the voltage of said source to effect a change in said first-named potential difference of a magnitude substantially' equal to the corresponding change in said Vsecondnamed potential difference, and adjustable electrical means for maintaining said two potential differences in substantial equality.

'7.,In photoelectric apparatus the combination with an electric lamp of a plurality of electrical resistors to form a Wheatstone bridge, a source of v power supplying said lamp and said bridge, means for adjusting said bridge to produce a potential difference functionally dependent upon the temperature of said lamp a light-sensitive device adapted to receive radiation from said lanip and to produce an electric current as a result thereof, a circuit for opposing said potential difference derived from said device to a potential difference functionally dependent upon the radiation from said lamp and means for adjusting the rate of Lchange of said first-named potential difference to with an electric incandescent lamp of a plurality of electrical resistors in an electrical network, a source of electric current for supplying said lamp and said network, variable means for adjusting said network to produce a potential difference subject to variation with changes in temperature of the filament of said lamp caused by fluctuations in the voltage of said source, a light-sensitive device adapted to be illuminated bysaid lamp and to produce an electric current as a result thereof, a circuit for opposing said potential difference due to the current produced by said device to a second potential difference subject to variation with changes in the brightness of said lamp caused by said fluctuations in voltage and regulating means for adjusting the rate of said first-named variation to substantial equality with the rate of said second-named variation to render the performance of said apparatus insensitive to said fluctuations in voltage.

9. 'I'he combination with a source of radiant energy of a plurality of electrical conductors in a Wheatstone bridge, a source of electrical current for said bridge, a receiver for said radiant energy, variable means for adjusting said bridge to produce a potential difference subject to variation with changes in the voltage of said source of current, a circuit for balancing said potential diil'erence against a second potential difference derived from said receiver and subject to variation with changes in the intensity of said radiant energy occasioned by said changes in voltage and regulating means for adjusting the functional dependence of said rst-named potential diilerence upon the voltage of said source of current in accordance with the functional relation which exists between said second-named potential difference and said voltage to render said changes in voltage Without effect upon the balance ln said circuit.

l0. In photoelectric apparatus the combination with an electric incandescent lamp of a plurality of electrical resistors of substantially zero coeillcient of resistivity in a bridge network, a source of electric current for said network, a photoelectric cell illuminated by said lamp and adapted to produce an electric current in consequence thereof, a variable resistor for adjusting said network to produce a potential difference dependent upon the temperature of said lamp and bearing a predetermined functional relation to the voltage of said source, a voltage-measuring circuit including a balance detector for opposing said potential difference to a second potential difference derived from said cell and bearing to the voltage of said source a functional relation similar to said rst-named functional relations, and means for maintaining said potential differences in substantial equality whereby the variations in the voltage of said source are without effect upon the indications of said balance detector.

1l. In a system including an energy-emitting element, a source of `electric current for said element and a receiver for said energy, the combination of a plurality of electrical resistors in a bridge network to produce an unbalance potential difference, a circuit for applying said potential.

difference to another element in said system and regulatory means for adjusting said network and said potential difference to produce an eilect upon said system which is equal and opposite to a similar effect produced upon said receiver by changes in voltage of said source, whereby the performance of said system is rendered independent of such changes.

12. Potentiometric measuring apparatus comprising a plurality of resistors of negligible temperature coefiicient and a resistor of substantial temperature coeilicient of resistivity mutually interconnected to form a Wheatstone bridge, a source of electric current connected to opposite points of said bridge, the resistance of said resistor of substantial temperature coefficient being dependent upon the voltage of said source, a circuit including a calibrated potentiometer connected to points of said bridge diagonally disposed with respect to said source of current and deriving current therefrom by virtue of said bridge being out of balance, and a circuit including a galvanometer for opposing to said potentiometer a potential difference to be measured, said bridge arms being so adjusted that a variation in voltage of said source of current is without effect upon the indications of said potentiometer, due to a concurrent compensating change in the resistance of said resistor of substantial temperature coeilicient.

13. Potentiometric measuring apparatus comprising a Wheatstone bridge having three arms of substantially zero temperature coelcient and a fourth arm of substantial temperature coefficient, a source of current connected to opposite points of said bridge, the resistance of said fourth arm being dependent upon the voltage of said source, a circuit including a calibrated potentiometer resistance connected to points of said bridge diagonally disposed with respect to said source of current, said circuit deriving current from said source through said bridge, a second circuit including a potential difference to be measured, means including a detector of balance for opposing said potential difference toa potential diilerence derived from said potentiometer resistance, means for maintaining equality between said two potential differences and regulatory means for adjusting the arms of said bridge to cause the variations in the resistance of said fourth arm induced by changes in the voltage of said source to counteract an effect upon said apparatus normally produced by said changes in voltage, thereby rendering measurements obtained with said apparatus independent of fluctuations in the voltage of said source of current.

i4. Photoelectric apparatus comprising the combination of a plurality of electrical resistors with an electric lamp to form a Wheatstone bridge, a source o current connected to opposite points of said bridge for supplying current to said lamp and to said bridge, the brightness of said lamp being dependent upon the voltage of said source, a circuit including a calibrated potentiometer resistance connected to points of said bridge diagonally disposed with respect to said source and deriving current therefrom by virtue of said bridge being out of balanceja second circuit including a light sensitive device electrically responsive to light emitted by said lamp and modified by said medium, means including a detector of balance for opposing a potential diierence'derived from said second circuit and dependent in magnitude upon the amount of light received by said light-sensitive device to a potential diiierence derived from said calibrated resistance, said bridge being adjusted to cause the current in said calibrated resistance to vary with changes in the voltage of said source in a ratio similar to the variations in said first-named potential diierence caused by changes in the brightness of said lamp induced by said changes in voltage and means for adjusting said two po tential differences to equality, the arrangement being such that said apparatus is rendered substantially insensitive to said changes in voltage of said source and responsive only to the property of the medium being measured.

15. An electric circuit including a thermionic tube comprising a heating element, an electronemitting element, an electron-receiving element and an element adapted to control electrically the flow of electrons from said emitting element to said receiving element, the combination of a `plurality of electrical resistors with said heating element to form a Wheatstone bridge, a source of electric current for supplying said heating element and said bridge, means for adjusting the arms of saidvbridge te produce an unbalance potential difference varying with the voltage of said source and with the temperature of said heating element and means for applying said potential difference to said control element to compensate for changes in emission from said electron-emitting element induced by said changes in voltage whereby the flow of electrons to said receiving element is rendered independent of the voltage uctuations of said source of current.

16. An electric circuit including a thermionic tube comprising a heater, a cathode, an anode and a control grid, the combination of a plurality of resistors of negligible temperature coefficient with said heater to form a Wheatstone bridge, a source of current connected to opposite points of said bridge and supplying current to said heater and to said bridge, electrical connections from said grid and said cathode to points diagonally disposed in said bridge from said source of current, and regulatory means for adjusting said bridge to produce a potential difference between said grid and said cathode varying with the voltage of said source and the temperature of said heater in such a manner as to compensate for concurrent variations in said potential difference and in the emission from said cathode induced by changes in voltage of said source, thereby rendering the ow of electrons to said plate and the current in said plate circuit substantially independent of changes in the voltage of said source.

17. The circuit of ciaim 1s in which the eatnode and the -heater are combined in a single element.

18. An electric circuit comprising a thermionic tube including a heater, a cathode, an anode and a control grid, the combination of three resistors of substantially zero temperature coeillcient with said heater to form a Wheatstone bridge, a fourth resistor connected between a point on said bridge and the circuit of said plate, a source of current connected across said fourth resistor and said bridge in series to furnish current to said heater and voltage to said plate, electrical connections from said cathode and the circuit of said grid to conjugate points of said bridge and means for adjusting said bridge to produce an unbalance potential diierence acting in said grid circuit and varying with the voltage of said source in such a manner as to maintain the current in said plate circuit substantially independent of the changes in voltage of said source of current.

19. The circuit of claim 18, in which the cathode and the heater are merged in a single element.

20. A balance detector for an electrical circuit comprising a thermionic tube including a heating element, a cathode, an anode and a control grid, means for connecting a circuit including said cathode and said grid to said electrical circuit to test the state of balance in said electrical circuit, a galvanometer in the plate circuit of said tube adapted to indicate changes in current flowing to said plate from said cathode, a plurality of electrical resistors interconnected with said heating element to form a Wheatstone bridge, a source of current for said heater and said bridge connected to opposite points of said bridge, electrical connections from said cathode and said grid circuit to points of said bridge diagonally disposed with respect to said source of current, and means for adjusting said bridge to produce in said grid circuit a potential difference varying with the voltage of said source by' virtue of resultant changes in the resistance of said heating element in such a manner as to compensate for concurrent variations in the emission from ,said cathode, thereby rendering the magnitude of the current ilowing in said plate circuit substantially independent of changes in the voltage of said source of current and responsive solely to the condition of unbalance which may exist in said electrical circuit.

21. The apparatus of claim 20 in which a three-element thermionic tube is used in which the functions of the heating element and the cathode are combined in one element commonly called the filament.

22. The combination with a calibrated potentiometer resistance of a bridge network vcomprising a plurality of electrical resistors of negligible temperature coemcient and a resistor of substantial temperature coeicient, an electric sourcel connected to opposite points -of said bridge supplying current thereto, the resistance of saidresistor of substantial temperature coeiicient depending upon the magnitude of said current, electrical connections from said bridge to said potentiometer adapted to supply current to said potentionmeter by virtue of said bridge 1 being out of electrical balance, a circuit including a detector of balance for opposing to said potentiometer a potential diierence to be measured and means for balancing said circuit, the arrangement being such that variations in the voltage of said source are substantially without effect upon the accuracy of said potentiometer due to concurrent compensating changes in the resistance of said resistor of substantial temperature coeiilcient occasioned by resulting changes in the magnitude of said current.

23. In an electrical system comprising a plurality of elements including a source of current the method of rendering said system insensitive to changes in voltage of said source which sonsists in combining a resistor of substantial temperaturev coeillcient with a plurality of resistors of negligible temperature coeilicient to form a bridge network, interposing said network between said source and the other elements of said system and adjusting the resistors of said network toproduce an unbalance potential diiference producing an eiiect upon said system which is equal and opposite to a similar effect produced within said system by said changes in voltage.

NORMAN E. BONN.V 

